Dry contact electrode with amplifier for physiological signals

ABSTRACT

An electrode and amplification circuitry connected thereto mounted within a housing for capacitively coupling physiologically produced potential along the skin surface of the body to the amplification circuitry without undesirable direct current shifts in potential and with rapid recovery from saturation due to a transient by means of a Zener diode or leakresistor circuit arrangement in the amplification circuitry.

' United States Patent 1191 Kaufman et al. a

1 11 3,744,482 1 1 Jul 10, 1973 DRY CONTACT ELECTRODE WITI-I AMPLIFIERFOR PHYSIOLOGICAL SIGNALS [75] Inventors: William M. Kaufman, ChevyChase;

Donald P. Powell, Baltimore, both of Md.

[731 Assignee: Hitt man Associates, Inc., Columbia,

[22] Filed: June 29, 1971 21 Appl. No.: 158,040

52 u.s.c1 ..128/2.06E,l28/2.1B,128/2.1E 51 1111 01. ..A61b 5/04 58 Fieldof 'Search 128/206 13,206 E, 1 128/20611, 2.1 E, 010. 4; 330/109 [56]References Cited UNITED STATES PATENTS 3,620,208 11 1971 Higley et'al...... 128 206 E 3,568,662 3/1971 Everett et a], 128/206 E PrimaryExaminer -william E. Kamm Attorney-Flei t, Gipple & Jacobson 1571 lABSTRACT 12 Claims, 8 Drawing Figures Pmmww 3.744.482

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I)RY CONTACT ELECTRODE WITH AMPLIFIER FOR PHYSIOLOGICAL SIGNALSBACKGROUND OF THE INVENTION The measurement of physiologically generatedelectrical potentials on the surface of the body is common in medicalpractice and in research. Two examples are electrocardiography (ECG) andelectroencephalography (EEG), both of which are frequently employeddiagnostically. Typically, electrodes are placed at various locations onthe surface of the body and the voltage between selected pairs ofelectrodes is measured (usually recorded) as a function of time.

If the electrodes in contact with the body are metallic conductors,various electrochemically induced electrical potentials can appearbetween the metal and the skin, for example, because of perspiration.This problem of contact noise with metallic electrodes necessitated thedevelopment of special conductive pastes in combination with specificmetals that would minimize noise generation at the electrode contact.The most popular combination of this sort consists of a silver metalcontact and a concentrated silver chloride aqueous solution in the formof a conductive paste.

Although the use of electrode paste minimizes the problem of contactnoise, there are several problems associated with the application ofpaste electrodes for long-term monitoring. When paste contact electrodesare used for many hours or several days continuously, skin irritation isa common problem. The continuous contact of the concentrated saltsolution on the skin is not fully acceptable and puffy irritated weltsmay arise. As the paste dries under the metal electrode, contact noiseis created which can become so severe that the electrodes must beremoved, cleaned and repositioned on the body in order to obtain anacceptable signal-tonoise ratio.

For these reasons, investigators have been pursuing the development ofcapacitively coupled electrodes. With typical capacitive coupling to thebody, the skin is in contact with a stable insulating'material, such asa metallic oxide, which is relatively chemically inert andnon-irritating. Such an electrode does not depend upon electricalconduction; therefore, the conductivity of the horny layer of skin andthe presence or absence of perspiration will not affect signal quality.Since the electrical impedance of a capacitive coupling increases withdecreasing frequency and since the frequency band of interest for mostbiological signals is very low (from as low as fractions of one hertz),it is necessary to provide an amplifier circuit with a very high inputimpedance. Previous investigators have described very high inputresistance dc amplifiers for this application and have mounted theseamplifiers in close proximity to the coupling insulator to minimizepickup of electromagnetic interference.

Unfortunately, zero signal stability of dc amplifiers is a major problemarea. Induction of a small charge on the control electrode of theinitial amplifying circuit element due to an input transient or aleakage current can cause the dc amplifier to shift its quiescentoperating point significantly. These effects make a dc amplifiersomewhat undesirable for circuit applications that do not require dcresponse capability. The pass bands that are used for the recording ofthe EGC and the EEG include low frequency components but do not includedc. Therefore, an ac amplifier could be suitable for these applicationsif properly designed and constructed for compatibility therewith.

SUMMARY OF THE INVENTION This invention relates to ac coupled electrodesfor the acquisition of electrical signals, particularly those potentialsgenerated physiologically which are normally measured at the surface ofthe body.

A primary object of the invention is the provision of dry contact meansinvolving a relatively inert material touching .the skin therebypreventing irritation and other medical complications at the skinsurface.

Another object of this invention is the elimination of electricalconduction with the skin so that the conductivity of the horny layer ofskin and the presence or absence of perspiration will not affect signalquality and so that direct current shifts in potential are avoided dueto capacitive coupling.

Still another object of this invention is the provision of meansfacilitating replacement of the coupling film.

A still further object of the invention is the provision of non-linearcircuit motion'action and high capacitive reactance to respectivelyallow rapid recovery from saturation due to a transient and enhancepatient safety by limiting Hz leakage.

BRIEF DESCRIPTION OF THE DRAWING The above and other objects of thisinvention will become apparent to those skilled in the art after adetailed description of preferred embodiments of this invention takentogether with the accompanying drawing wherein:

FIG. 1 is a side elevational'view of an insulated electrode;

FIG. 2 is a plan view thereof;

FIG. 3 is a side elevational view of an embodiment of the electrode;

FIG. 4 is a plan view of the embodiment;

FIG. 5 is schematic view of the capacitance coupling portion of theelectrode shown in FIGS. 1 and 2;

FIG. 6 is a schematic view of the capacitance coupling portion of theelectrode shown in FIGS. 3 and 4;

FIG. 7 is a schematic view of the electrode circuitry; and

FIG. 8 is a schematic view of an embodiment of the electrode circuitry.

DETAILED DESCRIPTION Referring in detail tothe drawing, there is shownin FIGS. 1 and 2 an electronic amplifier l0 housed within a cup 12formed of conducting material and extending below the bottom surface ofa molded casing 14 formed of insulating material. An insulating film 16is removably positioned over the exposed cylindrical wall and bottomsurface of cup 12 by means of a retainer ring 18. An electrical cable 20containing power and signal leads connects amplifier 10 to a powersupply and coupling filters (not shown) so that the electrode can beused to transmit the physiological signal to an instrument for visualdisplay or recording. The molding compound used for making cable 20,casing 14, and cup 12 into a compact signal unit is an insulatedmaterial such as epoxy or acrylic resins.

As shown in FIG. 5, insulating film 16 along the bottom surface of cup12 is placed in contact with the skin 30 of the subject being measured.The skin 30, being electrically conductive, serves as one of the platesof a coupling capacitor C and cup 12, which is preferably of stainlesssteel, serves as the other plate of C with film 16 being the dielectricmedium.

A second method of capacitively coupling the physiological signal is toeliminate the use of insulating film 16. Cup 12 is housed directlywithin casing 14 in a manner so that the bottom surface of cup 12 isflush with the bottom surface of casing 14 for exposure to the skin asclearly illustrated in FIG. 3. As shown in FIG. 6, the skin 30 and thebottom of conducting cup 12 form a terminal point 32 which is connectedto one terminal 34 of a conventional capacitor C which couples thesignal to the remainder of the electrode circuit.

The basic electronic circuit for the electrode employs an operationalamplifier A with an input coupling capacitor C and a capacitor C, in thefeedback loop. The resistance R, in the feedback loop sets the lowfrequency cut-off point of the pass band F, which is:

f I/Z'ITR C Operational amplifier A is a very high gain invertingamplifier with very high input impedance and very low output impedance.In the idealized limit, the amplifier gain and input impedance areinfinite and output impedance is zero. Micro-circuit operationalamplifiers are commercially available with gains on the order of 10input impedance on the order of ohms or greater, and output impedance onthe order of 10 or 10 ohms. A good example of such a commerciallyavailable device is theAmelco 2741 operational amplifier. Using theidealization defined above, the circuit response of amplifier A is:

15 /12, j21rf RFCFII j21rf RFCF using conventional electricalengineering notation and terminology. The amplitude of the frequencyresponse is:

zl i f F F f RFCF)2 which is a high pass response. The upper cutofffrequency is not apparent from this equation because of the idealizedassumption for the operational amplifier A. Since all realizableoperational amplifiers have an upper frequency limit to theiramplification, this will provide an upper limit to the pass band of theelectrode circuit. This upper limit is far beyond the frequency spectrumof physiological signals for typical commercial operational amplifiers.

Clinical quality ECG amplifiers must have a lower cutoff frequency of0.1 Hz or lower. Since practical coupling capacitors must be relativelysmall, a very high value for R, is needed. A coupling film ofcommercially available insulating material (e.g., 0.00025 inch Mylar)has approximately 2.5 X 10' [If capacitance per square inch.Conventional conductive electrodes are on the order of 0.25 to 1.00square inch in area. Therefore, a comparably sized coupling film willhave approximately 10* pf capacitance. The corresponding value of R forf0.1 Hz is R,= 1.6 X 10 ohms. This is a very high resistance value noteasily obtained with linear resistance material. It has been found insolving this dilemma that it is possible to use the reversecharacteristics of a semiconductor junction diode to obtain resistorswith this high level of resistance. FIG. 7 shows two diodes D connectedback-to-back so that reverse characteristics dominate for current ineither direction.

An improvement on this design is desirable because of the very lowfrequency response of the preamplifier. A low frequency responsecapability implies that the preamplifier will be sensitive to transientdc shifts. The decay time for recovery from a transient step input islonger, the lower the value of the low end cutoff frequency. A transientlarge enough to saturate the preamplifier and block signal transmissionmay last many seconds in a typical ECG monitoring application.

Clinical requirements are conflicting in that low frequency responsecapability is required for ECG, but a loss of ECG signal for more than afew seconds (typically 9 to 10 seconds) will strike an alarm. Thisproblem can be obviated by means of non-linear circuit action.

A non-linear circuit action is needed that will prevent the inputterminal of operational amplifier A (the summing point) from drifingeither positively or negatively in voltage beyond very narrowly definedlimits. For example, if the amplifier power supply is :9V and if theoperational amplifier gain is 10 then iQOuV at the summing point willsaturate the amplifier. Use of Zener diodes as back-to-back diodes Dzmaking up R, will provide saturation protection as shown in FIG. 8. Ifthe Zener voltage is considerably less than the voltage of the powersupply, then, as the amplifier output voltage increases in magnitudebeyond the Zener voltage level, diodes Dz begin to pass current readilythereby reducing the feedback resistance and hastening the decay of thetransient voltage. Once the outputfalls below the Zener voltage level,then amplifier A resumes linear operation.

Another means of obtaining this non-linear action is to add an inputleak resistor R to the circuit of FIG. 7. In this circuit R is aresistor whose value has been specially selected to be large enough soas not to affect the low end frequency response and small enough to leakoff the charge at the input once amplifier A goes into saturation. Inthe normal linear mode of operation, the effective value of any inputleak resistance is multiplied by the operational amplifier gain. Whenthe amplifier saturates, the normal high gain drops off and inputresistor R permits a relatively rapid discharge of the potential at thesumming point.

While preferred embodiments of this invention have been illustrated anddescribed, it should be understood by those skilled in the art that manychanges and modifications may be resorted to without departing from thespirit and scope of this invention.

I claim:

1. A dry contact electrode for receiving physiological signals from thesurface of the skin, comprising a casing, conducting means secured tosaid casing and adapted to form one plate of an input capacitor,amplifying means housed within said conducting means, coupling means forinsulating said conducting means from the surface of the skin andadapted to physically connect the surface of the skin to an input ofsaid amplifying means through said conducting means so that the skinforms the other plate of said input capacitor, said coupling meanscomprising inert material which eliminates skin irritation and othermedical complications arising from contact with the skin surface,wherein said amplifying means has feedback means comprising a feedbackcapacitor and a pair of diodes connected back-to-back in parallel withsaid feedback capacitor for providing high capacitive reactance andlimiting 4. The electrode of claim 1, including resistive meansconnected across the input of said amplifying means for leaking offcharge at the input of said amplifying means when going into saturation.

5. The electrode of claim 1, wherein said pair of diodes breakdown to alow resistance level at a predetermined voltage level hastening thedecay of a transient voltage allowing rapid recovery from saturation.

6. The electrode of claim 1, wherein said diodes are Zener diodesv 7. Adry contact electrode for receiving physiological signals from thesurface of the skin of a patient, the electrode comprising: a maincasing; conducting means supported by said main casing for defining oneplate of an input capacitor; insulating means associating with saidconducting means for insulating said conducting means from the skin of apatient in such a manner that when said dry contact electrode is incontact with the skin of the patient, the skin defines the other plateof said input capacitor; amplifying means housed by said main casing;circuit means for connecting said input capacitor to said amplifyingmeans; output means for transmitting the output signal developed by saidamplifying means to a load; feedback capacitance connected between aninput and an output of said amplifying means; and a pair of feedbackdiodes connected backto-back in parallel across said feedbackcapacitance.

8. The electrode of claim 7, and further comprising resistor meansconnected across the input of said amplifying means.

9. The electrode of claim 7, wherein said diodes are Zener diodes. g

10. The electrode of claim 7, wherein said insulating means takes theform of a film covering the outside surface of said conducting means.

11. The electrode of claim 10, and further comprising retaining meansfor removably mounting said insulating film over the outside surface ofsaid conducting means.

12. A dry contact electrode for receiving physiological signals from thesurface of the skin of a patient, the electrode comprising: a maincasing; conducting means supported by said main casing for contactingthe surface of the skin of the patient; amplifying means housed by. saidmain casing; capacitor means connected between said conducting means andan input of said amplifying means for transmitting physiological signalsto said amplifying means; output means for transmitting the outputsignals developed by said amplifying means to a load; feedbackcapacitance connected between the input and the output of saidamplifying means; and a pair of feedback diodes connected back-to-backin parallel across said feedback capacitance.

1. A dry contact electrode for receiving physiological signals from thesurface of the skin, comprising a casing, conducting means secured tosaid casing and adapted to form one plate of an input capacitor,amplifying means housed within said conducting means, coupling means forinsulating said conducting means from the surface of the skin andadapted to physically connect the surface of the skin to an input ofsaid amplifying means through said conducting means so that the skinforms the other plate of said input capacitor, said coupling meanscomprising inert material which eliminates skin irritation and othermedical complications arising from contact with the skin surface,wherein said amplifying means has feedback means comprising a feedbackcapacitor and a pair of diodes connected back-to-back in parallel withsaid feedback capacitor for providing high capacitive reactance andlimiting leakage, and an output of said amplifier means from whichamplified physiological signals can be extracted.
 2. The electrode ofclaim 1, wherein said coupling means includes an insulating filmcovering the outside surface of said conducting means.
 3. The electrodeof claim 2, including retaining means adjacent said casing andphysically engaging said insulating film for removably mounting saidinsulating film over the outside surface of said conducting means. 4.The electrode of claim 1, including resistive means connected across theinput of said amplifying means for leaking off charge at the input ofsaid amplifying means when going into saturation.
 5. The electrode ofclaim 1, wherein said pair of diodes breakdown to a low resistance levelat a predetermined voltage level hastening the decay of a transientvoltage allowing rapid recovery from saturation.
 6. The electrode ofclaim 1, wherein said diodes are Zener diodes.
 7. A dry contactelectrode for receiving physiological signals from the surface of theskin of a patient, the electrode comprising: a main casing; conductingmeans supported by said main casing for defining one plate of an inputcapacitor; insulating means associating with said conductiNg means forinsulating said conducting means from the skin of a patient in such amanner that when said dry contact electrode is in contact with the skinof the patient, the skin defines the other plate of said inputcapacitor; amplifying means housed by said main casing; circuit meansfor connecting said input capacitor to said amplifying means; outputmeans for transmitting the output signal developed by said amplifyingmeans to a load; feedback capacitance connected between an input and anoutput of said amplifying means; and a pair of feedback diodes connectedback-to-back in parallel across said feedback capacitance.
 8. Theelectrode of claim 7, and further comprising resistor means connectedacross the input of said amplifying means.
 9. The electrode of claim 7,wherein said diodes are Zener diodes.
 10. The electrode of claim 7,wherein said insulating means takes the form of a film covering theoutside surface of said conducting means.
 11. The electrode of claim 10,and further comprising retaining means for removably mounting saidinsulating film over the outside surface of said conducting means.
 12. Adry contact electrode for receiving physiological signals from thesurface of the skin of a patient, the electrode comprising: a maincasing; conducting means supported by said main casing for contactingthe surface of the skin of the patient; amplifying means housed by saidmain casing; capacitor means connected between said conducting means andan input of said amplifying means for transmitting physiological signalsto said amplifying means; output means for transmitting the outputsignals developed by said amplifying means to a load; feedbackcapacitance connected between the input and the output of saidamplifying means; and a pair of feedback diodes connected back-to-backin parallel across said feedback capacitance.